Known as color display devices there have been Braun tubes, projector system employing light-bulbs, liquid crystal devices, electro-luminiscent devices, etc. but their merits have almost invariably been accompanied by some sorts of disadvantages. Examples of such disadvantages are as follows:
The Braun tube requires a high voltage source and occupies a large space, and furthermore, the size of its screen is limited. The light-bulb arrangement also requires a large space. In the liquid crystal device, the thermal characteristic is not satisfactory and the visibility as well as the contrast is poor.
In the case of electro-luminiscence device, the brightness is inadequate and its life is not long enough.